The invention relates to a process for the desulfurization of a crude gas stream containing at least H.sub.2 S in a Claus plant, afterburning of the Claus waste gas, and gas washing (scrubbing) of the waste gas from the afterburning to remove SO.sub.2, in which case SO.sub.2 recovered by the gas scrubbing is recycled to a point before the Claus plant.
Claus plants are widely used to recover elementary sulfur from gas streams containing H.sub.2 S. In this connection, a part of the H.sub.2 S is combusted to SO.sub.2 in a Claus reactor. SO.sub.2 and any remaining H.sub.2 S are reacted to form elementary sulfur and water in catalytic/thermal reactors corresponding to the Claus furnace. Claus plants have proven themselves very well in practice. But the degree of desulfurization that can be achieved with a clean Claus plant (depending on design and operation, between 90% and 97% of crude gas sulfur) does not satisfy today's requirements for reducing sulfur emissions to the atmosphere. To increase the degree of desulfurization and thus to reduce sulfur emissions, additional plant facilities downstream from the Claus plant are necessary for further treatment of the Claus waste gas. This further treatment of the Claus waste gas is usually referred to as "tail gas treatment."
A known process variant calls, for example, for the continuation of the reaction of the bound sulfur in elementary sulfur, which is incomplete in the Claus plant. In downstream catalysts (additional catalytic Claus stages), the reaction to sulfur continues corresponding to thermodynamic equilibrium at a reduced temperature level below the dew point of sulfur (sub-dew-point process), and simultaneously elementary sulfur is adsorbed on the catalyst. In this way, several reactors are periodically regenerated or switched to adsorption. Only by careful adjustment of the purge gas composition during regeneration can early deactivation of the catalyst be avoided. Contaminated, sulfur-containing gases must, if they cannot be handled in the Claus plant, be disposed of in some other way. Sulfur components COS and CS.sub.2 carried by the Claus waste gas are hydrolyzed to H.sub.2 S and CO.sub.2 to only a small extent and thus increase the sulfur burden in the waste gases. Spent catalysts must be disposed of. With such processes, degrees of desulfurization of up to 99.6% are achieved with relatively low operating and investment costs or up to 99.8% are achieved with considerably increased costs.
Another variant (direct oxidation process) for further treatment of the Claus waste gas comprises the hydrogenation of all sulfur compounds that are also contained in the Claus waste gas into H.sub.2 S. H.sub.2 S that is produced is chemically bonded by an aqueous absorption agent in an oxidative scrubbing or separated in a water-separation stage with catalytic oxidation. The oxidative scrubbing makes a sulfur yield of 99.9% possible, but with very high operating and investment costs. In the case of the catalytic-oxidative process, the degree of desulfurization remains limited to a maximum of 99.8% due to thermodynamic factors.
The so-called recycling processes, which all together comprise a gas scrubbing, teach a third variant of the further treatment of Claus waste gases. In this case, however, the Claus waste gas must first be completely hydrogenated or oxidized. When the Claus waste gas is hydrogenated, the resulting H.sub.2 S is scrubbing out in a chemical absorption step. When the scrubbing agent is regenerated, H.sub.2 S is released in concentrated form and can be fed to the Claus plant. For the reduction of H.sub.2 S, gases with hydrogen, carbon monoxide, or gas mixtures consisting of these substances are used. The degree to which the sulfur compounds are reacted into H.sub.2 S depends on the activity of the catalyst. During reaction to H.sub.2 S, COS can occur in the presence of carbon--for example in the form of CO--which is to be reacted separately. The reaction to H.sub.2 S requires an excess of reducing gas, so that depending on the SO.sub.2 content of the Claus waste gases, afterburning (combustion) of the scrubbed gases is necessary. Since, in addition to H.sub.2 S, CO.sub.2 is also absorbed by the scrubbing agent, an CO.sub.2 /H.sub.2 S mixture is produced as recycled gas to the Claus plant. Due to the recycling of the recycled gas to the Claus plant, said plant must be suitably enlarged. The spent catalyst must be disposed of. In the way described, a satisfactory rate of desulfurization can be achieved, but at very high investment and operating costs. In the second case, the sulfur compounds contained in Claus waste gas are oxidized to SO.sub.2, and the waste gas from the (combustion zone) is provided to a downstream gas scrubbing to ensure SO.sub.2 removal, and SO.sub.2 recovered by the gas scrubbing is recycled to a point before the Claus plant. The Claus waste gas is combusted using a combustible gas that is to be provided for the afterburning stage. Drawbacks of this approach are the emission of CO.sub.2, CO and H.sub.2, as well as the accumulation of contaminated waste water and other residues, whose removal or working-up is associated with high costs.
A feature that is common to all previously described known processes is that they process the Claus waste gas of a Claus plant that is optimized and maximized for sulfur recovery.